In today's world, we face two major challenges: urban air pollution and the limited availability of fossil fuels. This prompts researchers to seek alternatives in sustainable fuels and renewable energy sources. In both fields, the automotive industry plays a significant role. To overcome these challenges, during the last decade, the car companies and automobile industries have endeavored to develop electrical and hybrid vehicles. There is a big challenge to designing high power-to-weight, specific energy, and energy density batteries for them. By this fact, rechargeable lithium-ion batteries could play an essential role in developing more efficient and reliable cars. The efficiency, safety, and life of these batteries are greatly affected by their operating temperature. In this study, to cool and standardize the surface temperature of 18650 lithium-ion batteries, three novel finned separator plates are applied in the fluid flow path inside the battery pack. The research delves into the effects of inlet velocity on the thermal efficiency of the battery pack at high discharge rates (3C, 4C, and 5C) using ANSYS-Fluent software, employing the k-ω SST turbulent model for analysis.